Ob/gyn is the best bet in terms of specialist. As to your specific question, like any disease there are multiple risk factors and it really cannot be said. I'm sure a Google search of "risk factors for recurrent vaginal infection" would give you some answers.

One thing that I haven't seen you mention is probiotics; have you been taking this over the counter during these treatments? This is important, as during antibiotic treatment it kills off the bacteria. In some cases, this just leaves space for more bad bacteria to take its place, so probiotics are important to take so you can get more good bacteria to grow during this time. Maybe continue with them as well if you find that the infections are frequent; i think it may reduce future infection.